Microbiology of Meats
Meat and meat products have been a constant food for man as far back as there has been any evidence of civilization on the face
of the earth. Prehistoric man’s crude hunting and eating implements which have come to light are evidence that he must have
consumed his share of dinosaur and other forms of mammalian flesh. Ancient drawings and records on rock and in the caves of
the primitive people further substantiate this fact.
General factors associated with the micro flora of meat: Microorganisms play an important role in the quality of the meat
before, during and after processing, by initiating many undesirable biological changes in the meat. In addition, the age of the
animal, type of ration, and sanitary surroundings very materially influence quality of the product. Many other factors, such as
the method slaughter, chilling of the carcass, and storage of the meat also influence the quality of the product. Obviously, some
attention should be given at this time to the general sources of micro-organisms in meats.
Unsanitary practices either in the abattoir (public slaughterhouse) or farm butchering may contribute many undesirable
micro-organisms to the meat. Bacterial contamination may also be due to contact with spoiled or diseased meat and finally
inadequate care in handling, storage, and processing of the meat.
Sources of Micro-organisms in Meat
There is an inherent microflora in normal animal tissue. In healthy tissue micrococci are present in varying numbers.
Since the animal’s body is not free of bacteria and since, after death bacteria can enter the tissues through the intestinal and
other mucosa, the carcass may have an inherent microflora.
The surface of meat may contain a variety of micro-organisms primarily to air contamination. No particular kind of organisms
are inherent on the surface of the meat.
Bacteria present in the muscle fibers and other parts of the care may be due to slaughtering practices or infection of the animal
prior to slaughter, such as Brucella, Salmonella, Streptococcus, and Mycobacterium tuberculosis, also certain anaerobic
bacteria may be present. Achromobacter and Pseudomonas are predominant in meat held at temperatures. Also the presence of
Bacilli, Staphylococci, and lactobacilli may contribute to surface slime.
Meats contain a few unsaturated fatty acids and in the presence of light, copper and iron, an oxidation may take place.
Achromobacter and some yeasts may produce lipases which may cause rancidity. Both chemical and microbial activity may
cause rancidity in meats.
Molds
As a rule, molds may cause considerable trouble in the meat packing industry, particularly with different kinds of preserved
meats. The growth of molds on hams may cause no pronounced decomposition. However, the presence of molds on the product
represents an economic loss by the removal or reconditioning of the meat. The occurrence of molds on meat products appears to
be mainly due to lack of care and sanitary practices and to excessive humidity maintained in the storage rooms.
Molds and yeasts find pickling vats a favorable environment and occur frequently in these vats if sanitary precautions are not
observed.
Aspergillus, Mucor, Penicillium and Rhixopus are the principal genera associated with meats.
Biochemical Changes in the Meat
Meats may undergo spoilage as a result of microbial action on the fats and proteins. Many organisms produce the enzyme lipase
which attacks the fat. Thus the presence of these micro-organisms in meat is significant in that they may bring about more or
less rancidity. Also, other microorganisms may form enzymes capable of producing significant changes in the protein.
Oxidizing organisms sometimes impart tallowy flavors to meats containing fat. Many kinds of organisms are able to initiate
hydrolysis of fats, but only a few varieties can oxidize fats directly.
Sometimes the fat assumes a pink color. This may be due to the growth of certain chromogenic organisms which produce a
soluble Pigment that diffuses through the fat.
Mechanism of Microbial Spoilage in Ham
How do spoilage organisms enter into the hams and other tissues of the carcass? This is a debatable question. Several
investigators, Topley and Wilson (1938), Arnold (1928), and Burns (1934) claim that tissues are not always free of organisms,
yet most of the early reports suggest that tissues of normal healthy food animals are sterile. A voluminous review of the
literature offers ample support for both viewpoints. Let us assume that contamination of the carcass is quite logical during the
slaughtering and dressing process.
According to Jensen ham sours may be grouped into 6 classes:
1. Shank sours occur in the tibial bone marrow and are due to bacterial growth, particularly those that can grow at 38°F.
2. Body sours or “loin sours.” These signify souring of the meat.
3. Aitchbone sours due to portions of the ospubis remaining on the ham.
4. Stifle-joint sours. This is the articulation of femur and fibulatibia.
5. Body-bone sours (femur marrow).
6. Butt sours (butt between aitchbone and muscle).
The term sour refers to putrefaction of the muscle and not the formation of acid.
Jensen has very clearly summarized the current theories regarding the causes of ham sours as follows:
1. Faulty exsanguination of the animal.
a. Not bled properly.
b. Severing of jugular vein too small to bleed out rapidly.
c. Too large an incision.
2. Improper chilling.
a. Frozen shanks cannot be properly pickled.
b. Slow chilling of the carcass.
3. Contaminated salt used in pickling.
4. Previously used contaminated brines.
5. Excessive mechanical injury by dehairing machine.
6. Injury or bruises during processing of hams.
7. Holding carcass too long at high temperatures.
8. Failure of salt to penetrate into marrows.
9. Insufficient salt used to inhibit bacterial growth.
10. Poor quality hogs.
The below table shows the effect of temperature on the rate of growth of micro-organisms.
GROWTH OF MICRO-ORGANISMS IN MEAT AT DIFFERENT TEMPERATURES
Temperature,
Degrees F
35
40
45
50
55
Per Cent Increase of Micro flora Over the
Initial Count at the End of Each Period
------------------------------Hours-------------------------------24
48
72
96
2
4
8
9
3
7
10
20
8
20
25
30
20
30
40
45
25
40
45
60
FACTORS THAT INFLUENCE MICROBIAL CONTAMINATION OF MEAT
1. Bacterial load in gut. Starvation for 24 hours recommended.
2. Physiological changes:
Animal excited or fatigued, bacteria enter tissue more readily.
Incomplete bleeding.
Glycogen used up in fatigue, pH will not drop to 5.5 from 7.2.
3. Method of killing and bleeding.
4. Rate of cooling.
5. Grinding meat influences bacterial count.
Microbial Content of Meat and Meat Products
Obviously, the number of organisms will vary widely due to external contamination of the carcass. The counts may range from
100 to 100,000 per gram of beef and 5,000 to 1,000,000 per gram of swine. The samples were taken from the surface of the
animal, which accounts for the wide variation in counts.
Chopped meat (hamburger) was examined by the author and showed a wide variation in counts ranging from 5,000,000 to
10,000,000 per gram of sample.
Minced or spiced meats showed a relatively low bacterial count due to the preserving action of the various condiments used.
The micro-flora in smoked or dried meats is usually very low due primarily to chemical and physical environmental conditions
which are detrimental to the growth of the organisms. Salting or brining of meats has about the same effect as smoking on the
number of micro-organisms present in the meat. However, halophilic (salt tolerant) bacteria may be able to thrive in a salt
environment. Canned meats are as a rule sterile. Meats held in cold storage temperatures may be of good quality after a long
storage period providing the product has been properly chilled and handled prior to freezing. Cold temperatures are not an
effective means for destroying organisms. They merely delay metabolism and reproduction of the organism. However, the
bacteria tend to die off slowly during the storage period. Psychrophilic bacteria are able to survive in cold storage temperatures
for relatively long periods of time.
Bacon is susceptible to certain microbial contamination. Lipolytic bacteria may cause rancidity when the bacon is held in
storage. Also sulfide forming bacteria may alter the color and produce an oxidized flavor. The presence of high concentrations
of nitrite may cause undesirable color changes.
Molds such as Aspergillas, Alternaria, Monilia, Mucor, Rhizopus, and Penicillium are encountered in fresh beef, pork sausage,
wieners and dried beef when temperatures, storage and humidity are favorable for their growth.